1. Field of the Invention
This invention in general relates to the field of leads for the electrical stimulation of living tissue, and more particularly, concerns a lead having improved means for anchoring the lead which result in a lead which is unusually resistent to dislodgment when placed within the epidural space.
2. Description of the Prior Art
Electrical stimulation of the body is an increasingly important medical procedure. In particular, electrical stimulation of the spinal cord has proven to be effective in relieving chronic pain. In this stimulation context, a lead is introduced into the epidural space which surrounds the spinal cord. With the patient under local anesthesia at the point of insertion, the lead is maneuvered by the surgeon until the position is obtained in which the lead provides the maximum relief to the particular patient. A problem encountered in many stimulating contacts and, in particular, in the context of stimulation of the spinal cord within the epidural space, is the dislodgment of the electrode after insertion. This problem arises from a number of factors, including: the lack of structure within the epidural space which might engage the lead and prevent it from moving; the fact that the dura and surrounding tissue of the spinal cord are extremely sensitive and considerable pain and possible permanent damage can result if this region is traumatized, making gross anchoring techniques such as penetration or fixation to the tissue are impossible; and the fact that 10 cm to 20 cm of lead may extend into the epidural space from the point of entry (at which point the lead can be firmly anchored just outside the epidural space). Dislodgment of the stimulation electrode tip after insertion may substantially lessen the relief the treatment provides, or make the treatment completely ineffective.
U.S. Pat. No. 4,044,774 issued Aug. 30, 1977 on an invention of Terry Corbin and Duane Zykovitz extensively discusses the techniques of spinal cord stimulation in the epidural space and the related problem of lead placement. This patent relates particularly to the structure within the lead body itself, and it does not appear to disclose anchoring apparatus external of the lead body.
U.S. Pat. No. 3,902,501 for an endocardial electrode issued Sept. 2, 1975 on an invention of Paul Citron and Eugene Dickhudt, U.S. Pat. No. 3,939,843 for a transvenous electrode, issued Feb. 24, 1976, on an invention of Nicholas P. D. Smyth, and U.S. Pat. No. 4,236,529 for a tined lead issued Dec. 2, 1980 on an alleged invention of Richard L. Little all relate to leads employing tines for anchoring an electrical stimulation lead. U.S. Pat. No. 4,236,529 cited above also discloses flattened tines; however, the tines are flattened in a direction parallel to the axis of the lead body, and the patent discloses that the purpose of the flattening is to make the tines thinner so that they are more flexible about the lead body axis. The above patents all disclose that the tines engage trabeculae within the body organ such as the heart, to provide lead anchoring. Since the epidural space does not contain trabeculae or other similar tissue, extension of the tined lead to the epidural space is not evident from these patents. U.S. patent application Ser. No. 926,100 filed July 19, 1978 on an invention of Alfred A. Iversen for a body stimulation lead discloses a body stimulation lead for use in the epidural space which employs a plurality of lobes preformed in the lead body to assist in maintaining the position of the electrode. U.S. Pat. No. 4,285,347 discloses another electrode employing lobes for anchoring an electrode in the epidural space. U.S. Pat. Nos. 3,866,615 and 4,154,247 disclose electrodes having lobes which are employed for anchoring electrodes in the heart.